As in former years, our studies have been primarily devoted to the reactions of transplantation antigens with humoral antibodies. Attempts have been made to prolong the survival time of renal xenografts by neutralization of naturally occurring species-specific antibodies in the recipient. The role of complexes composed of transplantation antigens and antibodies in inducing glomerulonephritis have been studied in order to obtain information about the possible role of such complexes in the formation of late glomerular lesion in renal allografts. Heterophile antibodies in human transplantation sera were studied by means of double diffusion in gel reactions against extracts of bovine erythrocyte stromata. Evidence was presented that these antibodies are engendered by transplantation antigens shared by human nucleated cells and erythrocytes of some foreign species. Skin reacting transplantation antibodies were further studied in rats and several properties of these antibodies were defined. Using cytolysis and mixed agglutination tests, serological detection of transplantation antibodies to antigens controlled by minor histocompatibility loci could be successfully achieved in man and mice. An observation was made inadvertantly that the Paul-Bunnell antigen is a murine T-cell marker. We recently developed a test of thrombocytolysis in agar gel by means of which several antigens of platelet cell surface including histocompatibility antigens may be detected. Our studies on man-mouse hybrid cell lines have been continued. We obtained consistently negative results in MLC with hybrid cells and allogenic lymphocytes indicating that the stimulating structure is absent from the hybrid cells even though they contain HL-A antigens. As a partial departure from the main line of investigation, studies on various autoantibodies to murine cell surface antigens were performed.